The present invention relates to a conductivity modulation type MOSFET (hereafter referred to as IGBT) and more particularly an IGBT with a structure in which the turn-off time is shortened.
Conventionally, the basic structure of an IGBT, as shown in FIG. 3, is a vertical structure with a P.sup.+ type substrate as a drain layer (collector layer) 1, an N.sup.+ type buffer layer 2, an N.sup.- type conductivity modulation layer (base layer) 3, a P-type base region 4 in an island shape, an N.sup.+ type source region (emitter region) 5 in an island shape, a gate oxide film 6, a polysilicon gate 7 and a source electrode (emitter electrode) 8.
When a positive gate voltage is applied, an n-channel is formed, and electrons flow into the conductivity modulation layer 3 of an N.sup.- type base from the N.sup.+ type source region. This electron flow, lowers the voltage of the conductivity modulation layer 3, thereby forward biasing a P.sup.+ N.sup.- junction on the drain side. As a result, positive holes flow into the N.sup.- type conductivity modulation layer 3 from the P.sup.+ type drain layer 1, and the resistance of the conductivity modulation layer 3 is lowered significantly. Thus, the on-resistance of the IGBT decreases.
During the turn-off period in which the gate voltage has been removed, the P-type base region 4 and the N.sup.- type conductivity modulation layer 3 are reverse biased, and the electrons are swept out to the side of the drain layer 1, while the positive holes are swept out to the side of the source region 5 by the enlargement of a depletion region. Thereafter, excess charges of electrons and positive holes that have accumulated and remain in the non-depletion region on the conductivity modulation layer 3 are reduced by means of recombination, thus reaching a thermal equilibrium state.
There are two known methods for shortening the turn-off time, in order to permit remaining electrons and positive holes to recombine quickly at the recombination center. One method produces crystal defects inside a semiconductor intentionally by irradiating the recombination center. The second method applies the doping of heavy metal atoms such as gold and platinum, using the resulting impurity center as the recombination center. For both techniques, a localized level is formed in the forbidden band and is used as a field for recombination, functioning as a life time killer.
In an IGBT with a vertical structure, however, the above-mentioned introduction method of the life time killer is applied uniformly to each layer in the vertical direction. Therefore, although the turn-off time is shortened, reduction of on-resistance which is a feature of the IGBT is diminished compared with the above. That is, although the irradiation produces a plurality of crystal defects on the two sides of the semiconductor substrate, it is difficult to introduce them locally into the vicinity of the non-depletion region in the conductivity modulation layer 3 even if the acceleration energy or the doping quantity is varied. Furthermore, although it is possible to control the diffusion depth by the diffusion temperature and the diffusion time, it is still difficult and impractical to introduce the crystal defects locally into the vicinity of the non-depletion region on a controlled basis.
Accordingly, it is an object of the present invention to provide a conductivity modulation type MOSFET in which a second buffer layer, which is to become a gettering region in the substrate structure, is formed in advance without positively introducing the life time killer. This way the gettering of heavy metal atoms progresses naturally. Thereafter, the second buffer layer will function as a local life time killer region as the result thereof, thus making it possible to realize a shortening of the turn-off time while maintaining a low on-resistance.